1. This invention relates broadly to shaped, self-supporting ceramic composites and to methods for producing the same. More particularly, this invention relates to shaped, self-supporting ceramic composites comprising a shaped preform infiltrated by a ceramic matrix; and to methods of making novel ceramic composites by infiltrating a shaped preform with a ceramic matrix by "growing" an oxidation reaction product from a parent metal precursor, which product embeds constituents of said preform thereby forming a composite having the geometry of said preform.
2. Description of Commonly Owned Patent Applications
The subject matter of this application is related to commonly owned and copending U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, now U.S. Pat. No. 4,851,375, which is a continuation-in-part of U.S. patent application Ser. No. 697,876, filed Feb. 4, 1985, now abandoned, both in the names of Marc S. Newkirk et al and both entitled "Composite Ceramic Articles and Methods of Making Same." These applications disclose a novel method for producing a self-supporting ceramic composite by growing an oxidation reaction product from a parent metal into a permeable mass of filler. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration.
The method of growing a ceramic oxidation reaction product is disclosed generically in commonly owned U.S. Pat. No. 4,713,360 which issued on Dec. 15, 1987 and was based on change U.S. application Ser. No. 818,943, filed Jan. 15, 1986, which was a continuation-in-part of Ser. No. 776,964, filed Sept. 17, 1985, now abandoned which was a continuation-in-part of Ser. No. 705,787, filed Feb. 26, 1985, now abandoned which was a continuation-in-part of Ser. No. 591,392, filed Mar. 16, 1984, now abandoned, all in the names of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods of Making the Same. This method of using an oxidation phenomenon, which may be enhanced by the use of an alloyed dopant, affords self-supporting ceramic bodies grown as the oxidation reaction product from a precursor metal. This method was improved upon by the use of dopants applied to the surface of the precursor metal as disclosed in commonly owned U.S. application Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filled Sept. 17, 1985, now abandoned, which is a continuation-in-part of Ser. No. 747,788, filed June 25, 1985, now abandoned which is a continuation-in-part of Ser. No. 632,636, filed July 20, 1984, now abandoned entitled "Methods of Making Self-Supporting Ceramic Materials," all in the names of Marc S. Newkirk et al. The entire disclosures of all of the foregoing Commonly Owned Patent Applications and patent are expressly incorporated herein by reference. It should be noted that U.S. application Ser. No. 822,999, was abandoned in favor of a Rule 62 Continuation Application (filed June 23, 1988; U.S. Ser. No. 220,935 now U.S. Pat. No. 4,853,352).
3. Description of the Prior Art
In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this intersect has been the superiority of ceramics with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities when compared with metals.
Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite structure typically comprises a ceramic matrix which encloses one or more diverse kinds of filler materials such as particulates, fibers, rods or the like,
The traditional methods of preparing ceramic articles involve the following general steps: (1) preparation of ceramic material in powder form; (2) grinding or milling of powders to obtain very fine particles; (3) formation of the powders into a body having the desired geometry (with allowance for shrinkage during subsequent processing), for example by uniaxial pressing, isostatic pressing, injection molding, tape casting, slip casting or any of several other techniques; (4) densification of the body by heating it to an elevated temperature such that the individual powder particles merge together to form a coherent structure, preferably accomplished without the application of pressure (i.e., by pressureless sintering), but in some cases an additional driving force is required and can be provided through the application of external pressure either uniaxially (i.e., hot pressing) or isostatically (i.e., hot isostatic pressing); and (5) finishing, frequently by diamond grinding, is required.
When these traditional methods are applied to the preparation of ceramic matrix composite materials, additional difficulties arise. Perhaps the most serious problems concern the densification step, number (4) above. The normally preferred method, pressureless sintering, can be difficult or impossible in preparing particulate composites if the materials are to highly compatible. More importantly, conventional sintering is impossible in most cases involving fiber composites even when the materials are compatible, because the merging together of the particles is inhibited by the fibers which tend to prevent the necessary displacement of the densifying powder particles. These difficulties have been, in some cases, partially overcome by forcing the densification process through the application of external pressure at high temperature. However, such procedures can generate many problems, including breaking or damaging of the reinforcing fibers by the external forces applied, limited capability to produce complex shapes (especially in the case of uniaxial hot pressing), and generally high costs resulting from low process productivity and the extensive finishing operations sometimes required.
Additional difficulties also can arise in the blending of powders with whiskers or fibers, and in the body formation step, number (3) above, where it is important to maintain a uniform distribution of the composite second phase within the matrix. For example, in the preparation of a whisker-reinforced ceramic composite, the powder and whisker flow processes involved in the mixing procedure, and in the formation of the body, can resulting non-uniformities and undesired orientations of the reinforcing whiskers, with a consequent loss of performance characteristics.
A method for producing metal oxide refractories by the oxidation/reduction ("redox") reaction of a metal with silica is disclosed in U.S. Pat. No. 2,702,750. According to this patent, a silica body is either submerged into a molten bath of a metal such as aluminum, or a metal powder is dispersed throughout the silica body and then heated. Where desired, an inert material, such as alumina, may be added to the body. The refractory product is produced by oxidizing the metal to its oxide while reducing the silica to liberate silicon. In U.S. Pat. No. 3,973,977, there is disclosed a method of making a cermet composed overwhelmingly of magnesium aluminate spinel by immersing an agglomerate of several oxides into a bath of molten aluminum. Neither of these two patents discloses the directional growth of an oxidation reaction product formed by oxidation of a metal precursor with a vapor-phase oxidant, nor do they disclose such growth and infiltration into a shape preform.
The Commonly Owned Patent Applications describe new processes which resolve some of these problems of traditional ceramic technology as described more fully therein. The present invention combines these processes with additional novel concepts to remove a further limitation of ceramic technology, namely, the formation of complex structures to net or near net shape. More particularly, the present invention provides for the formation of composite shapes having a relatively complicated geometry or configuration, for example, with contoured planes or surfaces and with bores or openings. Further, the present invention provides for fabrication of ceramic composites of certain predetermined geometry by an unusual oxidation phenomenon which overcomes the difficulties and limitations associated with known processes. This method provides shaped ceramic articles as one-piece bodies having a predetermined shape, and of a size and thickness which are difficult or impossible to duplicate with the presently available technology.